With increases in the cost of fossil fuels and a rise in public awareness of the environmental consequences of current fuel consumption habits, the demand for alternative, renewable energy sources is growing. One such renewable energy source is solar energy. It is estimated that approximately 99.9% of harvestable renewable energy is solar-based, which includes resources such as wind, wave power, hydroelectricity, biomass, and solar power.
To be useful, solar energy must be converted into a usable form. In most instances, solar energy is converted into electricity. A number of devices and methods are known for converting solar energy into electricity. These technologies can generally be characterized as active or passive and as direct or indirect solar energy-conversion systems. Active systems typically rely upon electrical and mechanical components to capture short-wavelength radiation in the form of sunlight and convert it into a usable form. Passive systems rely upon non-mechanical techniques to control the capture of sunlight and convert this energy into a usable form. Passive techniques include referencing the position of a building to the sun to enhance energy capture, designing spaces that naturally circulate air to transfer energy, and selecting materials with favorable thermal properties to absorb and retain energy. Direct systems typically convert sunlight into a usable form of energy in a single step. Indirect systems typically convert sunlight into a usable form of energy through multiple steps.
One way to actively convert solar energy into a usable form of energy is through the use of Concentrating Solar Thermal (CST) systems. Concentrating Solar Thermal systems generally rely upon a shaped reflective surface, known as a solar collector or solar concentrator, to concentrate sunlight. Solar concentrators receive solar radiation over a relatively large surface area and focus it on a relatively small surface. More specifically, solar concentrators use lenses or mirrors to focus a large area of sunlight into a small beam or plane. Most CST systems also incorporate tracking systems that allow lenses or mirrors to follow the path of the sun. Four common types of CST systems are the solar power tower, the parabolic dish, the solar bowl, and the solar trough.
Many types of solar troughs are well-known in the art. Examples of solar troughs are described in the following issued patents and printed publications, the disclosures of which are incorporated herein by reference in their entirety: U.S. Pat. No. 4,099,515 to Schertz; U.S. Pat. No. 4,243,019 to Severson; U.S. Pat. No. 4,296,737 to Silk; U.S. Pat. No. 4,313,422 to McEntee; U.S. Pat. No. 4,423,719 to Hutchinson; U.S. Pat. No. 4,493,313 to Eaton; U.S. Pat. No. 4,546,757 to Jakahi; U.S. Pat. No. 6,276,359 to Frazier; U.S. Pat. No. 6,832,608 to Barkai, et al.; U.S. Pat. No. 6,886,339 to Carroll, et al. U.S. Pat. No. 7,055,519 to Litwin; U.S. Pub. No. 2007/0034207 to Niedermeyer; U.S. Pub. No. 2007/0223096 to O'Connor, et al., and U.S. Publication No. No. 2007/0240704 to Prueitt.
Parabolic troughs generally have a long parabolic mirror with a tube, also known as a receiver, running the length at the focal point of the mirror. The receiver is filled with a fluid, such as, for example, water or oil. To maximize the reflectivity of the trough, the top surface of the mirror is usually provided with a silver coating or polished aluminum. Due to the parabolic shape of the mirror, the trough is able to concentrate reflected sunlight onto the receiver. The concentrated sunlight heats the fluid flowing through the receiver. Depending upon the type of fluid being used and the particular design of the trough, the temperature of the fluid can exceed 400° C. When the trough is incorporated as part of a CST system, the heated fluid is transferred to a power generation system and used to generate electricity. The process can be economical and can achieve thermal efficiency in the range of approximately sixty to eighty percent.
Parabolic troughs can occupy a fixed position or be adjustable. Since the amount of reflected to the receiver is a function of the angle of the sun in relation to the trough, the position of the trough in relation to the sun greatly affects the ability of the reflective surface to concentrate sunlight onto the receiver. When the sun is at a sharp angle in relation to the trough, such as in the early morning or late afternoon, the amount of insolation, or incoming solar radiation, that can be captured by the trough can be significantly reduced. Therefore, adjustable parabolic troughs are generally more effective and are preferred in the industry. Adjustable troughs can be designed to adjust their position with respect to the sun in various ways. For example, an adjustable trough can incorporate a sun-rotating mechanism that tracks the course of the sun.
Parabolic troughs that have the ability to track the sun are generally constructed so that their axis of rotation is parallel to the path of the sun as it moves across the sky. Current technology provides for continual automatic adjustment of the troughs that is coordinated with the sun's movement. Movement of the troughs in response to the changing position of the sun is generally accomplished through adjustments along an axis perpendicular to the axis of the troughs. Though east-west or north-south orientation of the collector axis is typically specified for year-round or summer-peaking sunlight collection, respectively, troughs can be oriented in any direction. The arrangement of troughs in parallel rows simplifies system design and field layout, and minimizes interconnecting piping. Parabolic troughs can also be mounted on the ground or on a roof.
Some solar collectors also have the ability to reflect short-wavelength solar radiation back into space. For example, U.S. Pat. No. 5,177,977 discloses a parabolic trough that can be defocused so that some of the short-wavelength radiation arriving at the mirrored surface of the collector is randomly directed back into space. A drawback of this feature, however, is the difficulty of interchanging between the configuration needed to concentrate sunlight onto a receiver and the configuration needed to redirect short-wavelength radiation back into space. In addition, there is a need to increase the efficiency of the redirection of short-wavelength radiation by parabolic troughs.
Since parabolic troughs depend upon a mirrored surface to concentrate reflected sunlight, environmental conditions that may reduce the reflectivity of the mirrored surface are of great concern. For example, inclement weather, dust, and wildlife can leave unwanted deposits on the inner surface of the trough that reduces the ability of the trough to reflect sunlight. To reduce the likelihood of damage to or dirtying of the reflective inner surface, some troughs can be rotated so as to achieve an inverted position. In the inverted position, the mirrored surface can be substantially shielded from hazards such as hail, dust, and other particulate matter. A drawback of these inversion capabilities, however, is that the parabolic shape of the trough requires the trough to be elevated high above the ground (or other mounting surface) so that edges of the parabolic structure will not strike the ground (or other mounting surface) when rotated or inverted. Specifically, building a support structure that is tall enough to accommodate inversion can substantially increase burdens associated with installing and placing solar concentrators to be elevated. Existing parabolic troughs also lack an effective and efficient way to clean deposited material from the mirrored surfaces.
In addition to the mirrored surfaces of parabolic troughs, the structure of the parabolic trough as a whole can be susceptible to damage by environmental forces such as high winds. Current construction techniques for building solar concentrators generally utilize materials having a high stiffness and that are rigidly joined together to form an uninterrupted parabolic trough. While this type of construction contributes to an efficient collection of sunlight, it can also lead to catastrophic damage or fatigue that ultimately results in failure. Specifically, the parabolic face of the trough acts as a wind barrier that places tremendous strain on the solar concentrator structure during periods of high wind. A procedure for reducing wind strain on the structure is to invert the parabolic shape of the solar collector. As with protecting the solar collector from deposits on the mirrored surface, a disadvantage of inverting the parabolic shape is that the trough must be sufficiently elevated above the ground (or other mounting surface) so that edges of the parabolic structure will not strike the ground (or other mounting surface) when rotated or inverted. Even when the parabolic shape of the solar collector is inverted, pressure differentials created by the movement of air over the inverted solar collector can produce structural strain that can reduce the life expectancy of the structure. In addition, while an elevated support structure may accommodate an inverted position, the increased height further destabilizes the structure.
Therefore, there remain opportunities to further improve upon current designs. What is needed in the industry is a display mount that improves upon the aforementioned drawbacks.